Tap holders of common types can usually only accommodate tools with shanks of a single dimension or have shank dimensions lying within a very tightly limited range. When it is therefore desirable to machine a workpiece with tools having different shank diameters, it has been necessary to change the entire tap holder. Thus it has been necessary for each machine to have several tap holders for tools with different shank dimensions. The assortment of tap holders has also increased due to each tool shank, e.g. those of threading taps, having two retaining surfaces, an upper quadratic section, and a circular intermediate section, which have different dimension relationships for different tools. It is, however, of importance that the tap holder grip is well suited to the retaining surfaces of the tool to obtain good power transmission to the tool and accurate working on the workpiece.
Some tap holders known per se involve an overload protection where an adjustable torque shall be transmitted between the drive body and a tool mounted in it, see for example the Swedish Pat. No. 324 095. If the tool were to fasten in the workpiece or the predetermined torque were exceeded for some other reason, power transmitting balls are pressed out of their normal positions and the power transmission between the drive body and the tool is interrupted.
The known construction is based on the tool shank being provided with grooves intended to coact with the power transmitting balls. This signifies that the known overload protection is only usable for tools with specially shaped shanks. Another disadvantage with the known overload protection is that the spring bias must be adjusted manually for each tool dimension, which is troublesome and extremely unsatisfactory, since it is difficult to achieve any definite graduation of the overload protection.